Among recordable optical discs, there is known an optical disc exploiting e.g. a phase change film. In these recordable optical discs, a spirally extending land(s) and a spirally extending groove(s) are formed. Of these, the grooves are used as recording tracks in which to record data.
Moreover, in these recordable optical discs, on the recording tracks or grooves of which data can be recorded, wobble signals or LPP (land pre pit) signals are prerecorded in edge parts of the recording tracks, that is, in boundary portions of the recording tracks with respect to the grooves.
The wobble signals are those signals recorded by meandering of boundary portions of the land(s) and the groove(s) at a preset period. The meandering shape is such that, when the disc is reproduced at CLV (constant linear velocity) or at CAV (constant angular velocity), the meandering has a preset constant period. For this reason, the wobble signals are used as clocks. The meandering frequency is sometimes modulated by e.g. addresses, in which case a reproduced signal is also used as address information.
The LPP signal is such a signal recorded by forming a pit in a portion of the land. With the LPP signals, the string of pits formed represents an address, such that reproduced signals are used as the address information.
The wobble signals and the LPP signals are detected from return light of laser light illuminated on the recording track (recording groove). In the return light of laser light illuminated on the recording track (recording groove), the wobble signal components and the LPP signal components are contained in the push-pull component (differential component) along the radial direction of the return light.
Specifically, the main laser light beam, illuminated on the recording track (groove), is detected by a four-segment photodetector 101 shown in FIG. 2. The four-segment photodetector 101 is split into two portions along a direction corresponding to the radial direction of the optical disc, while being split into two portions along a direction corresponding to its tangential direction. That is, the four-segment photodetector is split into four portions in a cross-shape. The wobble signal and the LPP signal are included in a differential component of the total light volume (A+D) of the photodetector segments A and D on one side, here on the outer rim side, of the photodetector segments divided along the direction corresponding to the radial direction, and the total light volume (B+C) of the photodetector segments B and C on the other side, here on the inner rim side, of the photodetector segments, that is, ((A+D)−(B+C)). This signal ((A+D)−(B+C)), representing the differential component, is referred to below as the radial push-pull signal.
Of course, these wobble and LPP signals need to be read out not only during readout of data from the recording track but also during recording the data.
For detecting the wobble and LPP signals during data recording, the reflected light of the laser light radiated for recording is detected to generate the radial push-pull signal.
With the phase change disc, for example, pits are recorded by radiating laser light as pulses. Hence, during recording, there is a timing during which pits are being formed on the recording track (write time) and a timing during which pits are not being formed on the recording track (bias time). Thus, in the return light during recording, the signal level during writing (pit level) and the signal level during bias time (read level) become higher and lower, respectively, as shown in FIG. 3.
For this reason, in detecting the wobble and LPP signals during recording, signal processing must be made in such a manner that changes in the light volume of return light during writing and those during bias timing will be sufficiently taken into consideration.
Meanwhile, there are occasions where, even though the light spot illuminated on the optical disc is illuminated on the center of a recording track, the center of the light spot of the return light is not coincident with the center position of the four-segment photodetector 101, as shown in FIG. 4.
There are also occasions where light volume distribution is not symmetrical but distorted relative to the center of the light spot illuminated to the four-segment photodetector 101.
Hence, a time averaged value of the total light volume of the outer rim side photodetector segments (A+D) when the four-segment photodetector 101 is divided into two portions along a line corresponding to the radial direction differs from a time averaged value of the total light volume of the inner rim side photodetector segments (B+C), such that the radial push-pull signal (A+D−(B+C)) is added by an offset.
Moreover, there is a marked difference in the power during the write timing and that during the bias timing. The result is that there is produced a large level difference between an offset E1 of the radial push-pull signal during writing, that is, the difference in the pit level during writing, and an offset E2 of the radial push-pull signal during bias timing, that is, the difference in the read level during bias timing, as shown in FIG. 5. That is, there is produced time change in the offset of the radial push-pull signal, as shown in FIG. 6.
These variations in the offset are conductive to deterioration of the slew rate or generation of ringing, when the detection signals A to D of the four-segment photodetector are transmitted to the downstream side circuitry, as a result of which reproduction characteristics of the wobbles or LPP signals, generated from the radial push-pull signal, are deteriorated.
These problems are felt more keenly when the data are recorded by multiple-speed recording on the optical disc.